Effect of number of receptors for gonadotropin-releasing hormone on the release of luteinizing hormone.

The relationship between number of receptors for gonadotropin-releasing hormone (GnRH) and the ability of the anterior pituitary gland to release luteinizing hormone (LH) was examined in ovariectomized ewes. A GnRH antagonist was used to regulate the number of available receptors. The dose of GnRH antagonist required to saturate approximately 50 and 90% of GnRH receptors in ovariectomized ewes was determined. Thirty min after intracarotid infusion of GnRH antagonist, ewes were killed and the number of unsaturated (i.e., those available for binding) pituitary GnRH receptors was quantified. Infusion of 10 and 150 micrograms GnRH antagonist over a 5-min period reduced binding of the labeled ligand to approximately 50 and 12% of controls, respectively. The effect of reducing the number of GnRH receptors on release of LH after varying doses of the GnRH agonist, D-Ala6-GnRH-Pro9-ethylamide (D-Ala6-GnRH) was then evaluated. One of four doses of D-Ala6-GnRH (0.125, 2.5, 50 and 400 micrograms) was given i.v. to 48 ovariectomized ewes whose GnRH receptors had not been changed or were reduced to approximately 50 or 12% of control ewes. In ewes with a 50% reduction in GnRH receptors, total release of LH (area under response curve) was lower than that obtained for controls (P less than 0.01) at the 0.125-micrograms dose of D-Ala (6.1 +/- 0.7 cm2 vs. 13.5 +/- 0.7 cm2) but was not different at the 2.5-, 50- or 400-micrograms doses of D-Ala6-GnRH.(ABSTRACT TRUNCATED AT 250 WORDS)